Large plastic panels can be produced by a number of techniques, including compression molding, ram extrusion, and sintering. Ram extrusion of granular polymer resins typically uses equipment comprising a ram and die arrangement. The polymer material to be extruded is introduced into an inlet end of a die and forward strokes of a reciprocating ram compact the material and force it through the heated die in a continuous manner to form an uninterrupted shape of a desired profile.
Many types of plastic materials such as polyvinyl chloride plastic and ABS plastic have been successfully extruded in this manner. However, this seemingly simple procedure has proven quite difficult in practice when employing ultra-high molecular weight (UHMW) polymer resins. Such resins are sensitive to shear forces, particularly when extrusion pressures are very high (approaching 50,000 psi), which is complicated by the fact that they tend to exert significant frictional forces on a die during the extrusion process. Further, these resins are sensitive to high temperatures, often showing degradation due to oxidation at temperatures above 260° C., so that dwell time within the die becomes important. Lowered temperatures that may permit longer dwell times, however, are not economical and often do not fully melt the resin.
Early attempts to extrude UHMW polymer resins often resulted in defects such as undesirable interior voids, cracks, or noticeable separations between various charges impelled by repeated strokes of the ram. Sometimes they even included regions which still exhibited the original powdery or granular structure of the material, having evidently been insufficiently softened in the die.
Recent advances in ram extrusion processes have provided for production of larger panels of material, but generally these panels are several inches thick and have limited widths. Unless the heating protocols used to produce these panels are tightly controlled, the extrusion can be misshapen, or the final dimensions may not be held within acceptable tolerances. Further, if air gains access to the compacted resin, and/or the temperature is increased too much, an oxidation process can start which lowers the molecular weight of the polymers and reduces the physical properties of the final product.
The production of wider thinner panels has also been limited by the ram extrusion apparatus. To produce a large flat panel of UHMW polymer resin, a die having a large top and bottom surface is employed. Such a die may experience increased internal pressures which are difficult to control, especially for panels having a thinner profile. In general, the large pressures cause the center region of these wider dies to bow, thus producing a panel having a greater thickness in the middle than the edges, which is more evident for thinner panels. Prior art attempts to solve this problem have used dies which are concavely bowed toward the center, to counter the large outward forces generated during the extrusion process.
Further, upon exit from the heated die, if the panels are not cooled evenly and at a uniform rate, the crystallinity of the extrudate becomes heterogeneous to such an extent that the resulting solid panel is not flat, stress free or possessing the homogeneous physical properties required for satisfactory end use of the UHMW polymer product. That is, the large surface area and high aspect ratio of such panels often causes them to distort upon cooling below the crystalline melt temperature external to the die. Differences in the rate of cooling and crystallization can cause warping, bowing, thickness and surface irregularities, and the like. Such defects would then require minimally, shaving of the surface and machining to size. However, some defects, for example warp and bow, may be impossible to remove.
Accordingly, there exists a need in the art for improved systems and methods for extrusion of UHMW polymer resins, and particularly, for extrusion of large aspect ratio panels of UHMW polymer resin.